In recent years, interest has significantly increased in the use of photons rather than electrons for conveying information. Some of the research in this regard has resulted in some special purpose machines and various combinations of electronic and optic elements However, efforts to configure optical computers have met with many of the same constraints encountered by early electronic computers. Useful components are still evolving and are not readily available commercially for the most part. Limitations on the number and kinds of devices for practical applications suggests reconsideration of the serial nature of early electronic computers and implementation along those lines.
For instance, some of the early serial computers used mercury delay lines for memory and later shifted to magnetic drums along with a small number of flip-flop circuits and a diode array. They followed a bit-serial approach because a parallel implementation of a given function such as an adder would require N times as many devices.
Electrooptic switches have developed which are optical guided wave devices produced by diffusing precisely placed titanium waveguides into high purity optical lithium niobate crystals. These are referred to as lithium niobate or LiNbO.sub.3 electrooptic switching elements. Such an element is capable of accepting dual light inputs (A and B), one to each light waveguide, and transmitting that light to the light waveguide outputs (D and E, respectively).
In the absence of an externally applied switching voltage at a control electrode (C) but with application of correct bias voltage, light entering at point A will couple to the neighboring waveguide and emerge at E. Conversely, light entering at B will emerge at D. This is known as the "crossed state". Whenever the correct voltage is applied to control electrode C, light entering at A emerges from its own waveguide at D whereas light entering at B emerges at its output E. This is known as the "bar state".
Commercially available such devices are several centimeters in length with appropriate optical fiber connectors. Specifications for a typical such commercial switch are: wavelength (lambda) of 1300 nm. switching speed of 0.3 ns., switching voltage of 5-8 volts, bias voltage of approximately 5 volts, an on-off cross-talk of less than 20 dB, and an insertion loss of under 7 dB. The switching is nonlinear and, while it does not exhibit complete saturation at maximum switching value, its sinusoidal characteristic does provide insensitivity to voltage in this region.